1. Field of the Invention
The present invention relates to a magnetoresistive random access memory, and relates to, for example, a reference cell of a magnetoresistive random access memory.
2. Description of the Related Art
A magnetoresistive random access memory (MRAM) makes use of a magnetoresistive effect element as a memory cell. The magnetoresistive effect element includes a free layer (recording layer) having a variable direction of magnetization, and a fixed layer having a fixed direction of magnetization. A non-magnetic layer is interposed between the free layer and the fixed layer. The magnetoresistive effect element is in a low resistance state when the direction of magnetization of the free layer is parallel to that of the fixed layer, and is in a high resistance state when the direction of magnetization of the free layer is antiparallel to that of the fixed layer. A difference in resistance state is used to record information.
Read of information is executed by causing a read current to flow through the magnetoresistive effect element, converting the resistance value of the magnetoresistive effect element to an electric current value or a voltage value, and comparing the electric current value or voltage value with a reference value. Write of information is executed by changing the direction of magnetization of the free layer of the magnetoresistive effect element by causing an electric current, which is spin-polarized by the magnetic moment of the fixed layer, to flow through the free layer. The polarity of data is controlled by the direction of the current flowing through the magnetoresistive effect element (document 1: U.S. Pat. No. 5,695,864). In this write method, compared to a method using a magnetic field, a more direct effect can be exerted on a nano-scale magnetic body. Thus, no erroneous write occurs in a neighboring memory cell, and high-speed reversal of magnetization can be expected. In addition, advantageously, the amount of electric current necessary for information write decreases as the memory size becomes smaller.
The determination of information, which is stored in the magnetoresistive effect element, is executed by comparing a cell signal, which is obtained by the magnetoresistive effect element, with a reference signal. Both the cell signal and the reference signal are current values or voltage values. In many cases, the reference signal has a value between a cell signal which is obtained by the magnetoresistive effect element that is in a high resistance state, and a cell signal which is obtained by the magnetoresistive effect element that is in a low resistance state. Various methods of generating the reference signal can be thought. For example, it is thinkable to use a reference cell. The reference cell has the same structure as the magnetoresistive effect element included in the memory cell.
Document 2 (U.S. Pat. No. 6,943,420), for instance, specifically describes a method of generating a reference signal. In document 2, four reference cells are used. The magnetoresistive effect elements of two of the four reference cells are fixed in a low resistance state, and the magnetoresistive effect elements of the other two reference cells are fixed in a high resistance state. Using these reference cells, a reference signal having a value between a cell signal at the time of the high resistance state and a cell signal at the time of the low resistance state is generated.
In addition, in document 2, the reference signal is generated by setting the direction of magnetization of the free layer of the reference cell to be perpendicular to the direction of magnetization of the fixed layer. According to this method, it is possible to realize a magnetoresistive effect element having a resistance value between a value in the high resistance and a value in the low resistance state of the magnetoresistive effect element.
In the technique of document 2, however, no consideration is given to the stability of magnetization configuration of the reference cell. As described above, the reference signal is a reference for determining the logic of the cell signal. Hence, if the reference signal is unstable due to an unstable magnetization configuration of the reference cell, the logic of the cell signal cannot correctly be determined, and the reliability of the magnetoresistive random access memory considerably deteriorates. Specifically, after the magnetization configuration of the reference cell is erroneously reversed, this reference cell becomes permanently defective.
If the reference cell is restored to the normal state by rewrite, the defect of the reference cell is eliminated. However, since the reference cell is a reference for a read operation, a mechanism for recognizing the state of the reference cell, that is, a mechanism for reading out the reference signal, is not provided in usual cases. The reason for this is that such a mechanism will hinder an improvement in integration density of the magnetoresistive random access memory. In addition, in a case where the magnetization configuration of a memory cell is erroneously reversed, this error can be corrected by an error correction circuit. However, such an error correction circuit is not provided for the reference cell.
Besides, document 3 (Jpn. Pat. Appln. KOKAI Publication No. 2006-210396) discloses that an MTJ element 62R is made to have a desired resistance value by adjusting the area of the MTJ element 62R of a reference cell RC.